XLocs and Gsa, the alpha subunit of the stimulatory G protein (Gsa), are two related proteins derived from the same gene locus, GNAS. XLas differs from the alpha subunit of the stimulatory G protein (Gsa) by a large N-terminal domain with unique sequence. It is otherwise identical to the latter and comprises most of the domains shown to be functionally important for Gsa. Thus, XLas is predicted to act as a novel alpha subunit of the stimulatory G protein. However, owing to its unique N-terminal domain, XLas could importantly differ from Gsa with respect to its biological roles and regulation. When introduced into cells endogenously lacking Gsa and XLas (Gnas^"'" cells), XLas can mediate receptor-stimulated cAMP generation, but receptor coupling to XLas could not be demonstrated in several other cell lines and tissues, suggesting that the actions of XLas can be cell-specific. The animal models in which XLas is ablated have established that XLas is essential for many biological processes, including postnatal adaptation to feeding, energy and glucose metabolism, and adipocyte biology. However, these in vivo studies were unable to demonstrate a role for XLas as a signaling protein that is similar to Gsa. Thus, the biological roles of XLas currently remain unclear. Our main objective is to determine the role of XLas as a novel alpha subunit that can replace Gsa in vivo. In Specific Aim 1, we propose to address whether XLas can substitute for Gsa in vivo by studying the actions of parathyroid hormone in the renal proximal tubule as an in vivo setting whereby the roles of XLos and Gsa can be studied precisely. In Specific Aim 2, we propose to determine whether XLas is involved in the temporal development of PTH-resistance as seen in patients with pseudohypoparathyroidism, a disorder caused by mutations disrupting Gsa activity and/or expression. For accomplishing our aims, we will investigate proximal tubular PTH responsiveness in various transgenic and knockout mouse strains generated by us or by our collaborators. These studies will reveal the interacting roles of XLas and Gsa in the proximal tubular PTH actions and are predicted to improve the understanding of hormone responses that typically involve G protein-coupled receptors. Ultimately, given the importance of this signaling pathway in normal physiology and human disease, our investigations may lead to the identification of novel targets for development of new drugs and diagnostic tools.